1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device which meets at least the performance requirements for a bright display with improved brightness and high contrast among the following performance requirements: a bright display with improved brightness and high contrast, a wide viewing angle in the vertical and horizontal directions on the display surface for excellent visual angle characteristics.
2. Description of Related Art
Generally speaking, liquid crystal display devices are classified into two types by display methods: a semi-transmission or transmission type with backlight and a reflection type. A reflection type liquid crystal display device is a liquid crystal display device which makes displays using only external light such as sunlight and illumination light without backlight. This type of liquid crystal display is widely used in devices which should be thin, lightweight and consume less power, such as personal digital assistants.
FIG. 5 is a sectional view which shows the general structure of a conventional typical reflection type liquid crystal display device. This is an example of a passive matrix STN liquid crystal display device.
This reflection type liquid crystal display device has a laminated structure consisting of a reflector 71 with a lower polarizer 70, a reflective mode STN (Super-Twisted Nematic) liquid cell 72 placed on top of the lowerpolarizer 70, a phase difference plate 73 placed on top of the cell 72 and an upper polarizer 74 placed on top of the plate 73.
The liquid crystal cell 72 generally comprises the following layers which are laminated in order from the lower polarizer 70 side: a lower glass substrate 75, a color filter 76, a lower transparent electrode layer 78, a lower alignment layer 79, an upper alignment layer 80 which is facing the lower alignment layer 79 with an STN liquid crystal layer 83 between the layers 79 and 80, an upper transparent electrode layer 81 and an upper glass substrate 82. There is an overcoat layer (not shown in the figure) of silica or acrylic resin between the color filter 76 and lower transparent electrode layer 78.
The above phase difference plate 73 compensates for phase difference in the light which passes through the STN liquid crystal to prevent the display from being bluish or yellowish.
Generally, the required display features of a liquid crystal display are excellence in the following aspects: (1) resolution, (2) contrast, (3) screen brightness, (4) color vividness, and (5) visibility (wide viewing angle, etc).
However, when a conventional reflection type liquid crystal display is used as the display of a personal digital assistant or similar device, on the display surface (screen), the area with good contrast in the horizontal direction is wide but the area with good contrast in the vertical direction is relatively narrow, which means high dependence on the viewing angle or poor visual angle characteristics. Also, the bright display (white display) is darker than in a transparent liquid crystal display with high luminance backlight.
One suggested approach as a solution to this problem is that the lower polarizer 70 located between the liquid crystal cell 72 and reflector 71 is omitted, namely only one polarizer (upper polarizer 74 located on the phase difference plate 73) is used to make the white display brighter at the time of application of selection voltage. However, in this type of reflection type liquid crystal display device, not only the bright display but also the dark display (black display) are brighter, causing a deterioration in contrast.
In view of the above circumstances, the present invention provides a reflection type liquid crystal display device which meets at least the performance requirements for improved brightness in the bright display (white display) and high contrast among the following performance requirements: improved brightness in the bright display and high contrast, wide viewing angles in the vertical and horizontal directions on the display surface for excellent visual angle characteristics.
To solve the above problem, the present invention provides a reflection type liquid crystal display which is characterized as follows. The reflection type liquid crystal display has a liquid crystal cell in which a transparent electrode and an alignment layer are provided on an inner side of one of two transparent substrates facing each other with a liquid crystal layer between them in the order of mention from the one substrate side while a transparent electrode and an alignment layer are provided on an inner side of the other transparent substrate in the order of mention from the other substrate side, and two phase difference plates and a polarizer which are provided on an outer side of the other transparent substrate in order from the other substrate side,
wherein the liquid crystal layer is twisted by 240 to 250 degrees in its thickness direction and the crystal cell has a retardation (xcex94ndLC) of 600 nm to 800 nm; and
wherein, viewing the alignment direction a of the alignment layer on the other transparent substrate side and the alignment direction b of the alignment layer on the one transparent substrate from the light incidence side, when a direction of a normal X is assumed to be a direction which lies between the alignment directions a and b and passes through both the intersection O of the alignment directions a and b and a direction angled by one half of an interior angle formed by the alignment directions a and b,
the retardation (xcex94ndRF1) of the phase difference plate adjacent to the other transparent substrate is from 100 nm to 200 nm, and the lagging axis xcex2 of the phase difference plate forms an angle (xcfx86RF2) of 60 to 100 degrees in the counterclockwise direction as viewed from the light incidence side with respect to the normal direction X,
wherein the retardation (xcex94ndRF2) of the phase difference plate adjacent to the polarizer is from 300 nm to 500 nm and the lagging axis xcex3 of the phase difference plate forms an angle (xcfx86RF2) of 90 to 140 degrees in the counterclockwise direction as viewed from the light incidence side with respect to the normal direction X, and
wherein the angle (xcfx86po1) formed by an absorption axis xcex1 of the polarizer with respect to the normal direction X is set at a value between 20 to 70 degrees or between 110 to 160 degrees in the counterclockwise direction from the light incidence side.
According to one aspect of the present invention, the reflection type liquid display device uses a single polarizer as it has a liquid crystal cell in which a transparent electrode and an alignment layer are provided on an inner side of one of two transparent substrates facing each other with a liquid crystal layer between them in the order of mention from the one substrate side while a transparent electrode and an alignment layer are provided on an inner side of the other transparent substrate in the order of mention from the other substrate side, and two phase difference plates and a polarizer which are provided on the outer side of the other transparent substrate in the order of mention from the other substrate side. With this structure, transmissivity is higher at the OFF time of voltage (when non-selection voltage is applied) and the bright display (white display) is brighter, which results in a higher contrast and excellent display characteristics.
Furthermore, the above liquid crystal layer is twisted by 240 degrees to 250 degrees in its thickness direction and the above crystal cell has a retardation (xcex94ndLC) of 600 nm to 800 nm;
when, viewing the alignment direction a of the alignment layer on the other transparent substrate side and the alignment direction b of the alignment layer on the one transparent substrate from the light incidence side, a direction of a normal X is assumed to be a direction which lies between the alignment directions a and b and goes through both the intersection O of the alignment directions a and b and a direction angled by one half of an interior angle formed by the alignment directions a and b,
the retardation (xcex94ndRF1) of the phase difference plate adjacent to the other transparent substrate is from 100 nm to 200 nm, and the lagging axis xcex2 of the phase difference plate forms an angle (xcfx86RF1) of 60 to 100 degrees in the counterclockwise direction as viewed from the light incidence side with respect to the normal direction X;
the retardation (xcex94ndRF2) of the phase difference plate adjacent to the polarizer is from 300 nm to 500 nm, and the lagging axis xcex3 of the phase difference plate forms an angle (xcfx86RF2) of 90 to 140 degrees in the counterclockwise direction as viewed from the light incidence side with respect to the normal direction X; and
the angle (xcfx86po1) formed by an absorption axis xcex1 of the polarizer with respect to the normal direction X is set at a value between 20 to 70 degrees or between 110 to 160 degrees in the counterclockwise direction from the light incidence side so that the white display (bright display) is brighter, leading to a higher contrast.
According to another aspect of the present invention, in the reflection type liquid crystal display device as described above, the phase difference plate adjacent to the other transparent substrate has a coefficient Nz of xe2x88x920.5 to 2.0 as expressed by the equation (1) shown below and the phase difference plate adjacent to the polarizer has a coefficient Nz of xe2x88x920.5 to 2.0 as expressed by the equation (1) shown below. This is preferable because it is ensured that the area with good contrast in the horizontal and vertical directions on the display surface is wide, and thus the viewing angle in the horizontal and vertical directions on the display surface is wide, resulting in excellent visual angle characteristics.
Nz=(nxxe2x88x92nz)/(nxxe2x88x92ny) . . . xe2x80x83xe2x80x83Equation (1)
where nx denotes a refractive index in an X-axis direction of the phase difference plate, ny a refractive index in a Y-axis direction of the phase difference plate, and nz a refractive index in a Z-axis direction of the phase difference plate.
According to another aspect of the present invention, in the reflection type liquid crystal display device which has any of the above-described structures, when the chromatic dispersion for the birefringence (xcex94nLC) of the liquid crystal constituting the liquid crystal layer is smaller than the chromatic dispersion for the birefringence (xcex94nRF1) of the phase difference plate adjacent to the other transparent substrate and that for the birefringence (xcex94nRF2) of the phase difference plate adjacent to the polarizer, the display provides higher contrast and excellent display characteristics.